Physics / core

Newton's Third Law

Also known as: action-reaction, N3

principle

For every action force, there is an equal in magnitude and opposite in direction reaction force. Newton's third law explains how rockets propel themselves in the vacuum of space, how you can walk by pushing backward on the ground, and why every interaction in nature involves mutual forces.

💡 Intuition

When you push something, it pushes back on you just as hard.

Core Idea

Forces always come in paired interactions — each force has a matching opposite force on the other object.

Formal View

If object A exerts a force \vec{F}_{A \to B} on object B, then B simultaneously exerts a force \vec{F}_{B \to A} = -\vec{F}_{A \to B} on A. The forces are equal in magnitude, opposite in direction, and act on different bodies.

🔬 Example

You push off the ground to jump; the ground pushes you upward.

🎯 Why It Matters

Newton's third law explains how rockets propel themselves in the vacuum of space, how you can walk by pushing backward on the ground, and why every interaction in nature involves mutual forces.

⚠️ Common Confusion

The action and reaction forces act on DIFFERENT objects, so they don't cancel out.

💭 Hint When Stuck

When applying Newton's third law, first identify the two interacting objects. Then name both the action force and reaction force, specifying which object each acts on. Finally, remember that the two forces are equal in magnitude, opposite in direction, and act on different objects — they never cancel each other.

Related Concepts

Next Steps

momentumimpulse

How Newton's Third Law Connects to Other Ideas

To understand newton's third law, you should first be comfortable with force and newtons first law. Once you have a solid grasp of newton's third law, you can move on to momentum and impulse.

Learn More

Introduction to Physics Concepts — In-depth guide

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Worked Examples

Step-by-step solved problems

Practice Problems

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Frequently Asked Questions

What is Newton's Third Law in Physics?

For every action force, there is an equal in magnitude and opposite in direction reaction force.

Why is Newton's Third Law important?

Newton's third law explains how rockets propel themselves in the vacuum of space, how you can walk by pushing backward on the ground, and why every interaction in nature involves mutual forces.

What do students usually get wrong about Newton's Third Law?

The action and reaction forces act on DIFFERENT objects, so they don't cancel out.

What should I learn before Newton's Third Law?

Before studying Newton's Third Law, you should understand: force, newtons first law.

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